Method of uniformly etching refractory metals, refractory metal alloys and refractory metal silicides

ABSTRACT

This invention is directed to a process for etching a semiconductor device using an etchant composition to form a predetermined etched pattern therein. The semiconductor device typically has a plurality of layers. At least one of the layers comprises a refractory metal, refractory metal alloy or refractory metal silicide. The etchant composition contains a high concentration of chlorine. The source (or TCP) power is decreased over that of conventional methods, and the bias (or RF) power is increased. Using such an etchant composition, along with the adjusted power levels, uniform etching and increased oxide selectivity is achieved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor devices and fabricationthereof. More particularly, the present invention relates to a method ofeffectively and uniformly etching refractory metals, refractory metalalloys and refractory metal silicides for use in semiconductor devices.

DISCUSSION OF THE BACKGROUND

In semiconductor fabrication, devices may be formed on a semiconductorwafer or substrate, which is typically made of silicon. Above the wafer,there may be disposed a plurality of layers from which the devices maybe fabricated.

To form the devices, a portion of the layers is patterned using asuitable etching technique and an appropriate etchant. Semiconductorprocessing makes extensive use of etching for active area definition,gate recesses, waveguide formation and so on.

Refractory metals such as, for example, molybdenum (Mo), titanium (Ti)and tungsten (W), along with their alloys and silicides, may be used inmanufacturing various semiconductor devices. As used herein, the term“refractory metal-containing material” refers to a material containing arefractory metal, a refractory metal alloy or a refractory metalsilicide. As used herein, the term “refractory metal-containing layer”refers to a layer of a semiconductor device that contains a refractorymetal, a refractory metal alloy or a refractory metal silicide.

Conventional technology for etching a layer containing a refractorymetal-containing material uses SF₆ or other fluorine-containing compoundsuch as CF₄ as the etchant, along with BCl₃ and possibly CF₄, Cl₂ andO₂. The process is operated at a high source (or TCP) power, typicallyabout 500 to about 600 watts, and a low bias (or RF) power, typicallyabout 70 to about 150 watts.

For example, U.S. Pat. No. 4,923,562 to Jucha et al. discloses anapparatus and a method for anisotropically etching refractory metalsusing a feed gas mixture that includes a fluorine source, a brominesource and an oxygen source.

Similarly, U.S. Pat. No. 5,853,602 to Shoji discloses a method of dryetching for patterning a refractory metal layer using a gaseous mixtureof SF₆/Cl₂/CO as the etching gas. The SF₆ and Cl₂ supply fluorine andchlorine radicals to etch the refractory metal layer, while the COproduces a reaction product that is deposited on the side surface of therefractory metal layer and prevents the fluorine and chlorine radicalsfrom etching the sides of the refractory metal layer.

U.S. Pat. No. 5,143,866 to Matsutani discloses a dry etching method forrefractory metals and their compounds using a mixed gas composition ofan etchant gas for etching the refractory metal and a deposit gas fordepositing the refractory metal such that the deposited refractory metalprotects the side walls of the refractory metal to be etched from sideetching. The deposit gas is a halide of the refractory metal that is tobe etched.

There are problems, however, with the conventional technology. Oneproblem is that the etch rate across the wafer is not uniform. Thedifferential in etch rate uniformity in the conventional process istypically about 20 to 30 percent. Such a large differential in etch rateuniformity requires a large amount of overetch time in order tocompletely clear or etch away the refractory metal-containing layerwithin the etching area.

Another problem with the conventional process is poor oxide selectivity.Selectivity refers generally to the ability of an etchant source gas todiscriminate between the different layers of the semiconductor devicethat may be exposed during an etch. Here, the oxide selectivity isexpressed as the ratio of the etch rate of the refractorymetal-containing material to the etch rate of the oxide. For a givenetch, an etchant source gas having a low oxide selectivity tends to etchaway at the oxide layer at a higher rate than an etchant source gashaving a high oxide selectivity. The poor oxide selectivity, which hereis about 1:1 or less, causes an oxide gouge as a result of the overetchstep in areas where the refractory metal-containing layer has alreadybeen completely etched away.

Yet another problem is that the existing technology cannot meet therequirement of a residual oxide layer of no less than about 200Angstroms in a M-I-M capacitor. A residual oxide layer of at least noless than about 200 Angstroms ensures reliability of the etchedcapacitor. The combination of the non-uniform etch rate and the pooroxide selectivity means that the existing technology is not capable ofsatisfactorily completing the etch (i.e., removing all the refractorymetal-containing material and leaving a minimum of no less than about200 Angstroms of residual oxide in the etched areas).

Yet another problem with the conventional process is that detection ofthe end point of the etch is not reliable.

In view of the aforementioned deficiencies attendant with the prior artmethods, it is clear that there exists a need for a method of etchingrefractory metals, refractory metal alloys and refractory metal suicideshaving a uniform etch rate and an increased selectivity to oxide.

SUMMARY OF THE INVENTION

Accordingly, one object of this invention is to provide a method ofuniformly etching refractory metals, refractory metal alloys andrefractory metal silicides.

Another object of the invention is to provide a method of etchingrefractory metals, refractory metal alloys and refractory metalsilicides that also has an increased selectivity to oxide.

To achieve the foregoing and other objects, and in accordance with thepurpose of the present invention as embodied and broadly describedherein, the present invention relates, in one embodiment, to a methodfor uniformly etching a semiconductor device comprising a plurality oflayers, wherein at least one of the layers comprises a refractorymetal-containing material. The method includes the step of at leastpartially etching the layers to an end point using a first etchantchemistry. There is also included the step of further etching the layersusing a second etchant chemistry.

In another embodiment, the present invention relates to a method foretching through a selected portion of a refractory metal-containinglayer and an oxide layer. The refractory metal-containing layer isdisposed above the oxide layer. The method includes the step of etchingat least partially through the refractory metal-containing layer using afirst etchant chemistry to an end point. The method further includes athe step of partially etching through the oxide layer using a secondetchant chemistry.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has now been found that a change or shift in the power distribution,i.e., the source power and the bias power, can improve etch rateuniformity. It has also been found that changing the etchant compositionfrom a fluorine-based composition to a chlorine-based compositionimproves oxide selectivity.

As used herein, uniformity of the etch rate may be determined using thefollowing formula:$\frac{{{maximum}\quad{etch}\quad{rate}} - {{minimum}\quad{etch}\quad{rate}}}{(2)\left( {{average}\quad{etch}\quad{rate}} \right)}.$

The process described in the present invention is for etching asemiconductor device to form a predetermined etched pattern therein. Theprocess comprises providing a semiconductor device having a plurality oflayers. At least one of the layers of the semiconductor device comprisesa refractory metal-containing material. The thickness of the refractorymetal-containing layer is about 1500 Angstroms. The refractorymetal-containing material may be a refractory metal such as, forexample, titanium, molybdenum or tungsten, a refractory metal alloy suchas, for example, TiW or a refractory metal silicide such as, forexample, tungsten silicide and molybdenum silicide.

In a preferred embodiment, the process of the present invention is usedto pattern a layer of a refractory metal-containing material that hasbeen deposited over a layer of oxide such as, for example, SiO₂. Thethickness of the oxide layer is about 500 Angstroms. The oxide layer maybe deposited on a typical silicon semiconductor wafer or substrate ormay be deposited on a layer of a refractory metal-containing materialsuch as, for example, TiW, aluminum, TiN or a tungsten alloy. Ifdesired, additional layers may also be present. The desired pattern ispreferably etched into the refractory metal-containing layer, stoppingon the oxide layer and leaving a residual oxide thickness of no lessthan about 200 Angstroms.

The etching of the semiconductor device comprising a plurality of layersis performed in two steps using two different etchant chemistries. Themethod includes the step of at least partially etching the layers withinthe etching area to an end point using a first etchant chemistry.Thereafter, the layers within the etching area are etched again, albeitwith a second etchant chemistry. This second etching step is preferablyallowed to proceed through any portion of the layers within the etchingarea not etched during the first etching step (i.e., in order to clearany remaining portion of the layer) and partially through the oxidelayer, typically for a time period of about 40 percent of the firstetching step.

A shift in the power distribution during the initial etching stepimproves etch rate uniformity. More specifically, the source powershould be lower than that used in the conventional process, and the biaspower should be higher than that of the conventional process. Typicalranges for the source power and the bias power, respectively, in the 15conventional process are about 500 to about 600 watts and about 70 toabout 150 watts. In the process of the present invention, the sourcepower is reduced and the bias power is increased and should be such thatthe ratio of bias power to source power is about 0.5-5. In the processof the present invention, the source power is between about 100 andabout 450 watts. Preferably, the source power is between about 125 andabout 210 watts and, most preferably, is between about 140 and about 170watts. The bias power is between about 200 and about 500 watts.Preferably, the bias power is between about 225 and about 350 watts and,most preferably, is between about 240 and about 270 watts.

Etching with the first etchant chemistry continues until the end pointis reached. The end point is the point at which the bulk of therefractory metal, refractory metal alloy or refractory metal silicidehas been etched away. The endpoint is detected using optical emission,more specifically, by measuring the percent change in the opticalemission intensity, as known to those skilled in the art. Typically,detection at 703nm is used. After the end point has been detected, thesecond etchant chemistry is employed to etch away the residual portionof the layers within the etching area, if any, and through part of theoxide layer before being terminated.

The layers located within the etching area are etched with an etchantcomposition to form a predetermined pattern therein. The etchantcomposition used in the present invention is chlorine-based. Unlike theprior art etchant compositions that are fluorine-based, the etchantcomposition used in the present invention contains a higher chlorinecontent than conventional etchant compositions. In other words, theetchant composition contains a high chlorine concentration instead of ahigh fluorine concentration. The amount of chlorine in the etchantcomposition is about 50 percent to about 95 percent, more preferablyabout 56 percent to about 90 percent, of the total gas flow of theetchant composition.

The etchant composition comprises a first etchant chemistry and a secondetchant chemistry. The first etchant chemistry comprises a source ofchlorine, typically chlorine gas. Other sources of chlorine such as, forexample, HCl and CCl₄ may also be used; however, BCl₃ may not be used asa source of chlorine. Compared with conventional etchant compositions,the flow volume of BCl₃ in sccm (“standard cubic centimeters perminute”) is reduced, preferably, to zero. BCl₃ may be used, though notas a source of chlorine, if one of the layers is aluminum. In any event,the flow volume of BCl₃ is much less than that used in the conventionalprocess.

To increase the etch rate, the first etchant chemistry further comprisesa small amount of a fluorine-containing material such as, for example,SF₆, F₂, NF₃ or CF₄. If desired, the fluorine-containing material may beleft out of the first etchant chemistry entirely.

Additionally, the first etchant chemistry may also comprise N₂.

By way of example, a mixture of Cl₂/SF₆/N₂ having a flow volume ratio insccm of 45:30:5 has been found suitable for use as the first etchantchemistry. Such a mixture uniformly etches through refractory metals,refractory metal alloys and refractory metal silicides.

Like the first etchant chemistry, the second etchant chemistry comprisesa source of chlorine, typically chlorine gas. Other sources of chlorinesuch as, for example, HCl and CCl₄ may also be used. Again, BCl₃ may notbe used as a source of chlorine, and the flow volume of BCl₃ in sccm ispreferably zero. Additionally, the second etchant chemistry may includeN₂. No fluorine is used in the second etchant chemistry because, whilenot wishing to be bound to any theory, it is believed that fluorineresults in a low oxide selectivity.

By way of example, a mixture of Cl₂/N₂ having a flow volume ratio insccm of 45:5 to 45:15 has been found suitable for use as the secondetchant chemistry. Such a mixture has an improved oxide selectivity andallows for stopping of the etch when a residual oxide layer having athickness of no less than about 200 Angstroms is reached.

In accordance with a preferred embodiment of the present invention, therefractory metal-containing layer is first etched with a first etchantchemistry. Etching of the refractory metal-containing layer terminateswhen it is determined that the bulk of refractory metal-containing layerhas been etched through. The first etchant chemistry comprises achlorine source and a fluorine source. The first etchant chemistry mayalso comprise N₂. Preferably, the chlorine concentration is greater thanabout 56 percent of the total gas flow.

Subsequently, the oxide layer, as well as any residual refractorymetal-containing material, is etched with a second etchant chemistry.Etching of the oxide layer terminates when the layer of residual oxideis no less than about 200 Angstroms thick. The second etchant chemistrycomprises a chlorine source. The second etchant chemistry may alsocomprise N₂. Preferably, the chlorine concentration is about 75 percentof the total gas flow.

The inventive etch process of the present invention may be performed inany of the known plasma processing apparatuses, including those adaptedfor dry etching, plasma etching, reactive ion etching, magneticallyenhanced reactive ion etching or the like. To further elaborate, in atypical plasma processing chamber adapted for dry etching, thesemiconductor wafer or substrate is treated with plasma. The chamberincludes an inlet port through which etchant source gases are suppliedto the chamber interior. A suitable RF energy source is applied toelectrodes associated with the chamber to induce plasma. The energyitself may be coupled inductively or capacitively to sustain the plasma,as is known. Species are then formed from the etchant gas source toreact with the wafer and etch away at the plasma-contacting layer of thewafer. The by-products, which may be volatile, are then exhaustedthrough an exit port.

Plasma etching relates to the situation where the wafer is positioned onthe anode, or ground electrode, during wafer processing. On the otherhand, reactive ion etching relates to the situation where the wafer ispositioned on the cathode, or powered electrode, during processing.Magnetically enhanced reactive ion etching represents a variant of thereactive ion etching geometry wherein a magnetic field is applied toreduce the loss of energetic electrons to the reactor wall surfaces.

It is contemplated that the invention may be practiced in any of theabove reactors, as well as other suitable plasma processing reactors.Note that the above is true irrespective of whether energy to the plasmais delivered through capacitively coupled parallel electrode plates,through electron resonance microwave plasma sources or throughinductively coupled RF sources such as helicon, helical resonators andtransformer coupled plasma.

In a preferred embodiment, the present invention is employed in a TCP™9600 SE plasma reactor, which is available from Lam ResearchCorporation. A mechanical clamp is used to hold the layers to be etched.As mentioned above, however, any other conventional and suitable plasmaprocessing systems may be employed. Particularly, a capacitor couplingreactor may also be used to practice the present invention. When such areactor is used, the bias power should be between about 100 and about750 watts, preferably between about 250 and about 350 watts. Such areactor does not use source power.

The process of the present invention results in a very uniform etch rateacross the wafer or substrate. Uniformity for the etch is advantageouslygood, at a low value of no more than about ±3.5 percent.

The process of the present invention also results in an improved oxideselectivity of about 3:1. The higher oxide selectivity is especiallyadvantageous in preventing excess oxide loss when the etch of therefractory metal-containing material is extended to ensure that all ofthe refractory metal-containing material is removed.

Other advantages of the process of the present invention includeaccurate determination of the end point during the main etch, a largeoperating pressure range (about 5-20 mTorr) and a tunable etch rate from3500 Angstroms per minute to 1500 Angstroms per minute, as opposed to2000±500 Angstroms per minute for the conventional process. The largeoperating pressure range is important because, in the conventionaletching technology, even a small change in pressure results in a changein uniformity. The large operating pressure range means that uniformityis not as sensitive to pressure changes as it is in the conventionaltechnology.

The invention will now be described by reference to the followingdetailed example.

The example is set forth by way of illustration and is not intended tobe limiting in scope.

EXAMPLE

A M-I-M capacitor device comprising a plurality of layers is made byfirst depositing a layer of SiO₂ having a thickness of about 500Angstroms on a semiconductor substrate. A layer of TiW having athickness of about 1500 Angstroms is then deposited on the layer ofSiO₂. The substrate is placed in a TCP™ 9600 SE plasma reactor and isheld together with a mechanical clamp. A first etchant chemistrycomprising about 45 sccm of Cl₂, about 30 sccm of SF₆ and about 5 sccmof N₂ is fed into the etching chamber. The pressure in the chamber iskept at about 11 mTorr. The source power is about 160 watts, and thebias power is about 260 watts. The TiW layer is etched until the endpoint is detected. At this point, a second etchant chemistry comprisingabout 45 sccm of Cl₂ and about 5 sccm of N₂ is fed into the etchingchamber. The pressure in the chamber is again kept at about 11 mTorr.The source power is about 400 watts, and the bias power is about 80watts. The oxide layer is etched for about 10 seconds. Under suchconditions, a uniform etch rate is seen and, in the second etch step,the oxide selectivity is about 3:1.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that changes and modifications can be madethereto without departing from the spirit or scope of the invention asset forth herein.

1. A method for etching a semiconductor device to form a predeterminedetched pattern therein, comprising: (a) providing a semiconductor devicehaving a plurality of layers, at least one of the layers of thesemiconductor device comprising a refractory metal-containing material;and (b) etching the semiconductor device under conditions with anetchant composition comprising a first etchant chemistry and a secondetchant chemistry, providing a uniformity across the plurality of layersof ± about 3.5 percent.
 2. The method of claim 1, wherein the refractorymetal-containing material is selected from the group consisting ofrefractory metals, refractory metal alloys and refractory metalsilicides.
 3. The method of claim 2, wherein the refractorymetal-containing material comprises a refractory metal selected from thegroup consisting of molybdenum, titanium and tungsten or a refractorymetal silicide selected from the group consisting of tungsten silicideand molybdenum silicide.
 4. The method of claim 2, wherein therefractory metal-containing material comprises TiW alloy.
 5. The methodof claim 1, wherein the first etchant chemistry comprises a chlorinesource and a fluorine source.
 6. The method of claim 5, wherein thechlorine source is selected from the group consisting of Cl₂, HCl andCCl₄.
 7. The method of claim 5, wherein the fluorine source is selectedfrom the group consisting of SF₆, F₂, NF₃ and CF₄.
 8. The method ofclaim 5, wherein the first etchant chemistry has a chlorineconcentration of about 50 percent to about 95 percent.
 9. The method ofclaim 5, wherein the first etchant chemistry further comprises N₂. 10.The method of claim 1, wherein the second etchant chemistry comprises achlorine source.
 11. The method of claim 10, wherein the chlorine sourceis selected from the group consisting of Cl₂, HCl and CCl₄.
 12. Themethod of claim 10, wherein the second etchant chemistry has a chlorineconcentration of about 50 percent to about 95 percent. 13-32. (canceled)33. A method of etching a refractory metal-containing layer and an oxidelayer, the method comprising: (a) etching the refractorymetal-containing layer to an end point using a first etchant chemistryat a bias power of from about 100 watts to about 750 watts, wherein thefirst etchant chemistry comprises a chlorine source and a fluorinesource; and (b) etching partially through the oxide layer using a secondetchant chemistry, wherein the second etchant chemistry comprises achlorine source.
 34. The method of claim 33, wherein the bias power isfrom about 250 watts to about 350 watts.
 35. The method of claim 1,wherein said refractory metal-containing layer is etched at a sourcepower of from about 100 watts to about 450 watts and a bias power offrom about 200 watts to about 500 watts.